Galaxy S5 G900V startup failure with inconsistent symptoms making troubleshooting difficult

My Samsung Galaxy S5 suddenly and quickly started deteriorating a few days ago. I did also install a software update from Verizon about a week ago, but what's odd is how the symptoms worsened gradually. (Why would software do that?) * It started with occasional, random power-offs. No worries, I just rebooted. * Later, some of those times I started seeing "SECURE FAIL: KERNEL" in red. 1 or 2 extra reboots, though, and I'd be good to go. * Yesterday, the phone shut off a few times, taking more reboots to get things going. * Now, sometimes the power button doesn't respond (not even the vibration). When it does, sometimes nothing shows up on the display. If a display shows, it might hang on the _Samsung GALAXY S5 "Powered by Android"_ loading screen—but that's not as common.* More common is simply going to that "SECURE FAIL: KERNEL" screen. I've been rebooting for an hour now—probably 20+ times—with no success in getting a full boot. * This screen actually says "Custom" as well, with an unlocked padlock icon. But I've never rooted a phone, nor done anything "custom." I read something about ART vs. Dalvik and that it's normal for this phone. Plus, that started showing up months ago, so I'm guessing it's unrelated. **Given the variety of symptoms—from the power button not responding to this "SECURE FAIL: KERNEL" message, and also the _gradual_ degrading of successful reboot attempts, am I correct to conclude that this is more likely a hardware issue than software, _e.g._ the update to Lollipop, if that's what it was?** Some other notes that may contain clues for the more experienced: - I did manage to get into recovery mode once, and wiped the cache. - The other two attempts left me with a screen saying "SECURE FAIL: RECOVERY". - Sometimes the phone seems to start up by itself after 10+ minutes of not being touched, having a black display, and no indicator lights. - While plugged in and in the above state, I don't see the green battery charging animation. Maybe that means it's not really "off" but stuck in some software block or loop. - Once I did see the battery animation, and the battery was at 88%, so I think the battery's okay. - This is a S5 G900V that I purchased new from Verizon in October 2015. I wish it would start up properly just _once_ so I can get some data off the phone's memory. --- An interesting development. While typing this post, it got past "SECURE FAIL: KERNEL", but froze at the Samsung logo animation. Then, the display went black, while the blue indicator light stayed on. A few minutes later, the phone restarted itself, passed the _Powered by Andoird_ screen as it usually does, and hit "SECURE FAIL: KERNEL." I rebooted again, and saw something I saw only once before, "SECURE FAIL: Modem."